An expanded polypropylene resin molded product is excellent in physical properties such as a shock absorbing property and a heat insulating property, and is used in various fields. For example, an expanded polypropylene resin molded product is used as a packing material, a buffer material, a heat insulating material, or a construction material. In particular, it is relatively easy to produce products having various shapes by use of an in-mold foaming molding method in which a mold is filled with expanded polypropylene resin particles and is heated with use of steam or the like so as to fuse the expanded polypropylene resin particles with one another and to consequently produce an expanded molded product having a predetermined shape. The in-mold foaming molding method is thus employed in various applications.
It is known that a polypropylene resin in-mold foaming molded product can have a shape such as a cuboid shape, a cylindrical shape, or a box shape. It is also known that an in-mold foaming molded product has a complicatedly-shaped part, such as (i) a rib of a container such as a box-shaped returnable delivery container or a storage container, the rib including a partition which partitions articles to be delivered or stored, (ii) open parts of a box-shaped returnable delivery container into which open parts a user puts his/her hands to lift up the box-shaped returnable delivery container, or (iii) a part of a main material for an automobile bumper in which part a through hole is intentionally formed so that a fog lamp etc. is provided in the through hole.
For such a complicatedly-shaped part, a thin wall part whose thickness is further reduced is often required and designed. It is difficult to fill, with expanded polypropylene resin particles, a portion of a mold for use in in-mold molding which portion corresponds to the thin wall part. This causes a deterioration in surface property of the thin wall part of an in-mold foaming molded product to be produced. Specifically, a gap between expanded polypropylene resin particles on a surface of the thin wall part of the in-mold foaming molded product is widened. This makes convexoconcave of the surface prominent. Moreover, expanded polypropylene resin particles poorly fuse with one another at an edge part of the thin wall part in which edge part surfaces intersect with each other. A ridge line of the edge part is not smooth but is uneven. In a case where the edge part is rubbed with a finger, the expanded polypropylene resin particles of the edge part peel off. In a case where the in-mold foaming molded product is employed as a container such a box-shaped returnable delivery container or a storage container, particles which have peeled off will adhere to an article to be stored or will be mixed with the article as a foreign material in the container. This problem is really serious in a case where a precision machine etc. is stored in the container.
For example, Patent Literature 1 discloses an in-mold foaming molded product which is produced with use of expanded polypropylene resin particles that contain (i) a mixture of specific two kinds of polypropylene resin and (ii) petroleum resin and/or terpene resin so that the in-mold foaming molded product has a thin wall part whose surface appearance is satisfactory.
However, the two kinds of polypropylene resin of Patent Literature 1 are limited to a polypropylene resin that contains butene-1 as a comonomer. The in-mold foaming molded product including the polypropylene resin is excellent in rigidity but poor in shock absorbing property and cushion property. Therefore, the in-mold foaming molded product is not suitable to be employed as a material for a container such as a box-shaped returnable delivery container or a storage container which container is used to deliver a fragile article. In order to improve such a poor shock absorbing property and cushion property, if a propylene/ethylene copolymer that contains no butene-1 is employed as the polypropylene resin of the in-mold foaming molded product, it is possible to easily attain a satisfactory shock absorbing property and cushion property. In this case, however, surface appearance of a thin wall part is deteriorated, and/or an edge part of the thin wall part peels off.